Radiation sensitive strontium sulfate and dosimeters employing same



y 1956 J. H. SCHULMAN ETAL 2,757,291

RADIATION SENSITIVE STRONTIUM SULFATE AND DOSIMETERS EMPLOYING SAME Filed April 29, 1953 INVENTORS JAMES H. SCHULMAN ROBERT J. GINTHER United States Patent RADIATION SENSITIVE STRONTIUM SULFATE AND DOSIIWETERS EMPLOYING SAME James H. Schulman and Robert J. Ginther, Washington, D. C., assignors to the United States of America as represented by the Secretary of the Navy Application April 29, 1953, Serial No. 352,068 '8 Claims. (Cl. 250-83) (Granted under Title 35, U. S. Code (1952), see. 266) This invention relates to a new class of materials which are sensitive to ionizing radiation, more particularly to X-rays and gamma rays. It also relates to new radiation dosimeters employing the new radiation sensitive material.

It is an evident matter that the treatment of individuals or the disposition of objects which have been exposed to X-rays or gamma rays can be expedited and in general more eflicaciously carried out with the aid of some means which would provide a record of the intensity of the received radiation which is susceptible of evaluation in terms of unit dosages.

The present invention has an object therefore the provision of means of the aforesaid kind which on exposure to X-rays or gamma rays is capable of developing a visible signal therein, the strength of which is indicative of the intensity of the absorbed radiation and can be evaluated in terms of unit dosages. It is a further object to provide means of this kind which can be made in a small size so as to be carried about on the person. Other objects will become apparent as the description of the invention proceeds.

The invention will be described with reference to the accompanying drawing in which are shown several forms of a dosimeter by which the new radiation sensitive material may be applied for the detection and recording of the intensity of X-rays and gamma rays and in which:

Figure 1 shows one type of dosimeter in which the radiation sensitive material in powder form is afiixed to a fiat support.

Figure 2 shows a modified form of dosimeter in which the radiation sensitive material in fine particle form is embedded in a solid transparent medium of matching refractive index, and a protective case therefor in exploded view, and

Figure 3 shows a further modified form of dosimeter in which the radiation sensitive element is a suspension of fine particles of the radiation sensitive material in a transparent liquid of matching refractive index filling a sealed flat-bottomed tube of transparent material provided with a window at its top end through which visual determination of the depth of the developed color may be made, and a protective case therefor in exploded view.

We have found in accordance with our invention that radiation sensitive materials which on exposure to X-rays or gamma rays are capable of undergoing development of color therein, the presence of which is indicative of exposure to said rays and the depth of which can be availed of to measure the dosage or intensity of the absorbed radiation, can be obtained by diffusing a small proportion of an alkali metal sulfate into the crystal lattice of strontium sulfate. The alkali metal sulfate acts to sensitize the strontium sulfate so as to cause it to dehave the property of developing color on exposure to X-rays and gamma rays.

The alkali metal sulfate may be, for example, sodium sulfate, potassium sulfate, etc. Preferred sensitizers are sodium sulfate and mixtures of the same with potassium sulfate. Calcium and barium sulfate may be present in solid solution with the strontium sulfate in proportions up to about 10 mol percent without any appreciable adverse effect on the sensitizing action of the alkali metal sulfate. Barium sulfate because of its known greater response to gamma ray absorption may be used to increase the sensitivity of the strontium sulfate for the detection and intensity measurement of gamma radiation.

The radiation sensitive materials or scotophors are prepared by intimately mixing pure strontium sulfate with between about 0.01 and 5 mol percent, preferably with about 1 mol percent, of the alkali metal sulfate both in powder form and firing the mixture at a high temperature to effect diffusion of the alkali metal sulfate in the crystal lattice of the strontium sulfate. An intimate mixture of the salts can be obtained by mixing the strontium sulfate with an aqueous solution of the alkali metal sulfate, evaporating to dryness, for example at C., thoroughly grinding the dried mixture and firing the resultant powder in silica crucibles or trays at a temperature of 8001300 C. for a period of about /2 to 4 hours. After firing of the salt mixture is completed, the mass is broken up, ground to a powder and any excess of the alkali metal sulfate leached out with water or other appropriate solvent. Sodium and potassium activated scotophors made in the foregoing way by the addition of 1 mol percent and more of the alkali metal ion and firing of the salt mixture at 1050 C. for 30 minutes are extremely sensitive to radiation, the color changing from white to magenta. Best sensitization for radiation was effected by the simultaneous incorporation of both sodium and potassium into the crystal lattice of the strontium sulfate for which one can employ sodium and potassium sulfate in varying relative proportions, for example, in 1:1 mol ratio. The presence of halogen ions and trivalent metal ions in the radiation sensitive strontium sulfate should be avoided since they diminish its capactiy to develop color on irradiation. For this reason the strontium sulfate and alkali metal sulfate should be free from these ions and the firing atmosphere uncontaminated with halogen.

The radiation dosimeters of the invention may take various forms in which the fine particles of the radiation sensitive material are supported by means which allows visual determination of the depth of the color developed in the sensitive material on exposure to the radiation.

A dosimeter for determining the intensity of the softer of the hard radiations, such as low voltage X-rays, is shown in Figure 1. A thin layer of the powdered radiation sensitive material 1 is affixed by means of a suitable adhesive, such as bone glue and synthetic resin adhesives, to a firm support 2, rigid or semi-rigid, such as a sheet of cardboard, metal, glass or plastic. A direct reading of the depth of the color developed in the radiation sensitive material in terms of unit dosage of radiation can be had by the use of a color chart 3 graded in depth of shade to correspond to units of, for example, 100, 200, 300, etc.. roentgens. The color chart is disposed adjacent the radiation sensitive material as shown in the drawing and can be printed on the surface of the supporting sheet or afiixed thereto in the form of a printed strip of paper or of similar flexible material.

For measurement of the intensity of the harder radiations such as high voltage X-rays and gamma rays, a longer optical path is needed than is provided by the dosimeter of Figure 1. For this purpose the powdered radiationsensitive material is dispersed in a transparent solid or liquid of matching refractive index to simulate in transparency a single crystal of large dimensions, for example, one in which the dimensions are 1" x l" x l". The transparent solid or liquid should have a refractive index within the range of 1.624-1.627 to match example, as the transparent liquid for suspending the particles of the radiation sensitive material.

The fine radiation sensitive material may be used in concentrations of from about 1 to 80% by weight in the transparent suspending medium. As will be understood by those skilled in the art, the concentration of the radiation sensitive material in the dosimeter will vary in accordance with the level of the intensity of the hard radiation to be measured, the higher the intensity of the radiation, the lower generally will be the concentration of the radiation sensitive material required. For example, with high intensity radiation of the order of a million or so roentgens about 1 to by weight of. the radiation sensitive material will sufiice for the development of the color in the dosimeter, whereas with low intensity radiation, higher concentrations of the radiation sensitive material will be required, on the order of 70 to 80% by weight. As will be also understood, the length of the dosimeter as it relates to viewing of it by transmitted light will depend upon the solids content therein, the higher this is the shorter is the permissible maximum length consistent with obtaining clarity for viewing the dosimeter by transmitted light. Dosimeters of a high degree of clarity containing 1 to 10% solids by weight can be made up in lengths of about a half inch, an inch or two inches, or in any practical longer length. For visually read dosimeters used for measuring low intensity radiation and containing 70 to 80% solids, the length of the dosimeter should generally not exceed about an inch.

The powdered radiation sensitive material may be dispersed in the solid or liquid transparent medium in any way found suitable. The dispersion can be effected by simply mixing the powdered radiation sensitive material in thorough fashion with the liquid transparent medium and with the solid transparent medium previously made thinly fluid by melting with heat. The dispersion from the latter operation is solidified in molds of the desired configuration which provide a pair of opposed fiatsurfaced end walls on the resulting dosimeter. The purpose of the opposed flat end walls is tofacilitate reading of the dosimeter by transmitted light. The liquid type of dosimeter is also constructed to have a pair of opposed fiat end walls for this purpose.

A dosimeter having a solid type radiation sensitive element for the detection and measurement of hard radiation is shown in Figure 2. The radiation sensitive element 4 is a suspension of fine particles of the radiation sensitive strontium sulfate in a transparent block of plastic of matching refractive index. The block is provided with a pair of opposed fiat surfaces to facilitate reading of the same by transmitted light. Surrounding the sensitive element 4 in tightly fitting relationship is a translucent color chart 5 formed of adhesively joined or fused longitudinal segments each uniformly dyed in a graded depth of the color developed in the sensitized strontium sulfate after irradiation. The color depths may be graded to correspond to unit radiation dosages of, for example, 50, 100, 150, 200, 250, 300, etc., roentgens. The translucent material of the chart 5 is the same as that of the radiation sensitive element 4 but in. place of the radiation sensitive material it contains a like proportion of the fine, untreated, non-sensitized strontium sulfate dispersed therein. A snugly fitting two part casing, 6 of radiation transparent plastic or of other similar durable material is provided for protecting the sensitive element and its associated color chart against chipping and breakage in handling and use. The dosimeter is read by removing the sensitive element 4 and its associated color chart 5 from the casing 6 and viewing the sensitive element 4- by transmitted light through its opposed fiat surfaces. By comparing the observed depth of color with the color values of the chart 5, the unit intensity of the hard radiation can be directly determined.

A dosimeter in which the radiation sensitive element is of the liquid type is shown in Figure 3. A tube 7 having a flat bottom and formed of a transparent material, such as a clear glass or plastic, is filled with the liquid radiation sensitive material 8 which is a suspension of fine particles of the radiation sensitive strontium sulfate in a liquid of matching refractive index. The tube 7 is provided at its upper end with a closure 9 for sealing it against outflow of the liquid. This closure is conveniently a clear plastic cap which is fitted tightly down over the end of the tube and shaped to give a transparent flat surface 10 thereacross for the purpose of facilitating viewing of the dosimeter by transmitted light. A two part closely fitting case 11 of radiation transparent plastic or of other similar durable material is provided to protect the dosimeter against damage in handling and use. The unit dosage of the absorbed hard radiation can be determined by viewing the exposed dosimeter through the transparent flat end surface 19 in transmitted light and comparing the observed depth of color with those of a series of colorometric standards graded by depth of color in units of radiation dosage, for example, 50, 100, 150, 200, etc., roentgens. The color standards are made up of the same transparent liquid as is used in the dosimeter but in place of the radiation sensitive material, a like proportion of the fine, untreated, non-sensitive strontium sulfate is dispersed in the liquid. The gradations in color can be obtained by dyeing portions of the liquid suspension to graded depths of the color developed in the dosimeter by the hard radiation.

It is to be understood that while we have described our invention with particular reference to dosimetry as practiced by visual observation, evaluation of the depth of the color developed in the exposed radiation sensitive material and translation of the reading into units of radiation dosage can also be practiced by the application of photometers or other fine optical metering devices.

While in the foregoing we have described certain specific embodiments of the invention, it is to be understood that these are intended primarily by way of illustration and the scope of the invention is to be limited only by the prior art and the appended claims.

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

What is claimed is:

1. A radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays comprising strontium sulfate having an alkali metal sulfate diffused in the crystal lattice thereof.

2. A radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays comprising strontium sulfate having sodium sulfate diffused in the crystal lattice thereof.

3. A radiation. sensitive material capable of developing color therein on exposure to Xrays and gamma rays comprising strontium sulfate having sodium sulfate and potassium sulfate diffused in the crystal lattice thereof.

4. A radiation dosimeter comprising fine particles of a radiation sensitive material capable of developing color therein on exposure to- X-rays and gamma rays supported by means allowing visual observation of the developed color, said radiation sensitive material comprising strontium sulfate having an alkali metal sulfate diffused in the crystal lattice thereof.

5. A radiation dosimeter comprising fine particles of a radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays supported by means allowing visual observation of the developed color, said radiation sensitive material comprising strontium sulfate having sodium sulfate and potassium sulfate diffused in the crystal lattice thereof.

6. A radiation dosimeter comprising a sheet of firm material having afiixed to a broad face thereof a thin layer of fine particles of a radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays, said radiation sensitive material comprising strontium sulfate having an alkali metal sulfate diffused in the crystal lattice thereof.

7. A radiation dosimeter comprising fine particles of a radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays dispersed in a transparent solid of matching refractive index having a pair of opposed flat surfaces, said radiation sensitive material comprising strontium sulfate having an alkali metal sulfate difiused in the crystal lattice thereof.

8. A radiation dosimeter comprising fine particles of a radiation sensitive material capable of developing color therein on exposure to X-rays and gamma rays suspended in a transparent liquid of matching refractive index which fills and is sealed within a flat-bottomed tube of transparent material provided at its top end with a flat transparent window through which visual determination of the depth of the developed color can be made, said radiation sensitive material comprising strontium sulfate having an alkali metal sulfate diffused in the crystal lattice thereof.

References Cited in the file of this patent UNITED STATES PATENTS 2,258,593 Black Oct. 14, 1941 2,624,011 Stern Dec. 30, 1952 FOREIGN PATENTS 17,666 Great Britain Sept. 24, 1903 

1. A RADIATION SENSITIVE MATERIAL CAPABLE OF DEVELOPING COLOR THEREIN ON EXPOSURE TO X-RAYS AND GAMMA RASY COMPRISING STRONTIUM SULFATE HAVING AN ALKALI METAL SULFATE DIFFUSED IN THE CRYSTAL LATTICE THEREOF. 